This disclosure relates in general to free-space communications systems and, but not by way of limitation, to retroreflector modulation of incident light for communication amongst other things.
Modulating retroreflectors provide a self-aligned return signal for a downstream data link in free space optical communications, such as satellite-to-ground, ground-to-satellite, and ground-to-ground transmission of optical data and signals. The optical power demands for the upstream and downstream signal are satisfied at the base station, permitting the remote station to communicate with greatly reduced power demands.
In such communications systems the signal from the remote station to the base station occurs without broadcasting the signal over a large area. Only receivers within the line of sight between the remote station and the base station can receive the return signal. This narrow return communications window, permits the remote station to remain quiet to others while successfully communicating to the base station.
Modulating retroreflectors have been designed with various technologies such as using a multiple quantum well modulator (MQW) or a liquid crystal (LC) modulator with a solid retroreflector or mechanically tilting the mirrors, and defocusing or diffracting reflected light in a hollow retro reflector. While such systems have been shown to work, they have their limitations such as high fabrication costs associated with MQW modulators and the slow modulation LC modulators. While the modulation schemes used with hollow retroreflectors perform quite well the hollow retroreflector has a limited field of view for receiving an incident beam of light.